In the last twenty years the rated power of wind turbines has gradually increased by enlarging the diameter of their rotors, which in turn need taller towers. The increased height may imply that the tower should essentially comprise several sections throughout its entire height, placed one on top of the other to form the tower and which are in turn transportable by road or rail. For example, to build an 100 m-high tower, 5 20 m-sections stacked on top of each other may be used, such dimensions being transportable by road and rail.
Moreover, to ensure that the towers, although higher, are equally stable and rigid, one option is to increase the transverse dimensions of the tower gradually from the top to the base of the tower. This increase may involve new problems for the transportation of the sections and a common solution is to divide them into longitudinal modules. The dimensions of the longitudinal modules allow their transport by road or rail.
Logically, increasing the rated power of the wind turbines leads to an increase in weight and dimensions of all wind turbine components in general, so the following aspects are particularly relevant in connection with assembly costs:
Tower height;
Rotor diameter and weight of the blade-hub assembly;
Weight of nacelle and subcomponents;
Weight of tower sections.
Specifically, the weights of the full tower sections of a 3 MW wind turbine can exceed 200 t which places high requirements on assembly cranes.
The use of these cranes is expensive, first, because of availability problems entailing high daily rental costs: there is no large park of such cranes; and second, the high costs associated with their transport due to the large number of trucks required to move them. According to data shown in U.S. Pat. No. 8,011,098B2, the cost of renting a crane for tower assembly can amount to $80,000 per week, plus nearly $100,000 for transport (forty trucks or more).
Such high costs justify the pursuit of alternative means to build wind turbine towers. Various procedures that can be followed for the assembly of these towers include:                Assembling the dowels one by one onto the rest of the installed tower, which is a problem when the dowels are not freestanding. This is the procedure used by other ATS (“antenna tower structure”) manufacturers. This process requires a large number of elevations and a complicated process for the positioning and securing of the dowels until a whole section is formed on the one below it, for the subsequent execution of the vertical joints and prior to stacking the dowels of the next section.        Pre-assembling complete sections by joining the dowels, executing vertical joints between them on the ground near the base of the tower and subsequently mounting the sections one on top of another. This procedure greatly facilitates tower assembly, as most of the operations are conducted on the ground. This is the procedure of U.S. Pat. No. 7,765,766B2. The assembly process comprises two stages:                    Section preassembly stage, in which the dowels that form a section are assembled at the base of the tower and the vertical joints are executed between concrete dowels, and            A lifting and piling stage in which the already pre-assembled sections are placed one on top of another.                        
An object of the present invention is to provide a method of assembling concrete towers which reduces crane requirements for the first procedure, while simplifying the overall assembly process with respect to the second procedure.
Furthermore, the prior art shows that the concrete tower is often a conical section tower, particularly frustoconical to withstand the high torque in the base. Furthermore, in some tower designs dowel height is much greater than tower height. This means that when the tower is conical, the dowels are unstable when placed upright, because the horizontal projection of the centre of gravity is outside of the perimeter defined by the base of the dowel.
These drawbacks are solved with the invention described below.